1. Field of the Invention
The present invention relates to wireless networks, and in particular to quickly identifying mixed mode packets and accurately determining the receiver offset when receiving mixed mode packets.
2. Related Art
The IEEE 802.11-2007 is a set of standards relating to wireless local area networks (WLAN). The legacy standards, e.g. 802.11a and 802.11g, have data rates that are relatively low. For example, both 802.11a (released 1999) and 802.11g (released 2003) have a data rate of 54 Mbit/s. In contrast, one of the most recent standards, 802.11n (projected release 2008), has a data rate of 300 Mbit/s. Thus, 802.11n is characterized as a high throughput protocol. The 802.11n protocol achieves this high throughput by transmitting and receiving using multiple chains (multiple-input multiple-output (MIMO)).
An 802.11 access point (AP) can operate in one of three modes: the legacy, mixed, or Greenfield mode. In the legacy mode, the AP can use one of the legacy protocols, e.g. 802.11a or 802.11g. In the mixed mode, the AP can use one of the legacy protocols or 802.11n. In the Greenfield mode, the AP can only use 802.11n (wherein “Greenfield” refers to a project that lacks any constraint imposed by prior work).
In a legacy mode, the preamble of the packet can be used for both coarse and fine timing to correct for receiver offset. This legacy preamble consecutively includes a short training field, two long training fields, and a signal field. The short training field can be used to roughly estimate the boundary between the short training field and the first long training field. This rough estimate can be characterized as “coarse” timing. In contrast, the two long training fields in the legacy preamble can be consecutively used to provide better timing information. Therefore, the two long training fields can provide fine timing. Note that after a boundary between any two fields in the legacy preamble is accurately established, the beginning of the data field in the legacy packet can be ascertained.
In line-of-sight (LOS) channels or small delay spread channels, in the frequency domain, the amplitude of a signal can be assumed to be flat across the sub-carriers. For example, FIG. 1A illustrates an idealized amplitude that is flat across 64 sub-carriers for a 20 MHz signal (i.e. −32 to 31). Note that the idealized phase across these same sub-carriers should also optimally be flat, as illustrated in FIG. 1B.
If there is a timing offset, then the amplitude may still be flat, but the phase will experience a linear phase shift over the sub-carriers (also called tone dependency). The phase can be calculated using the following equation:
                    ⅇ                                            -              j2Π                        ⁢                                                  ⁢                          n              e                        ⁢            k                    N                                    Equation        ⁢                                  ⁢        1            where N is the total number of sub-carriers (e.g. 64), ne is the number of slots a sample is shifted (which can be negative or positive, typically in the range of 1-3), and k is specific sub-carrier (e.g. between −32 and 31). Thus, as evidenced by the variable k, the phase is sub-carrier dependent. Moreover, ne/N is the “slope” or the rate of phase change. Notably, the timing offset determines the slope and vice versa.
A mixed mode packet is introduced to support multiple chain transmission, where both a legacy header and a high throughput header are repeated on all chains. For example, FIG. 2 illustrates an exemplary mixed mode packet format 200 that includes a legacy header 210, a high throughput (HT) header 211, and an HT data field 209. Legacy header 210 includes a legacy short training field 201, first and second legacy long training fields 202 and 203, and a legacy signal field 204. HT header 211 includes first and second HT signal fields 205 and 206, an HT short training field 207, an HT long training field 208 (note that although only one field is shown in FIG. 2, multiple HT long training fields may be used for multiple streams).
To avoid unintentional beam-forming effects (which would otherwise prevent all receivers from hearing the preamble), different amounts of cyclic shift can be applied to the legacy header and the HT header transmitted from different chains. In a simple cyclic shift, samples from one chain are rotated by one sample in a wrap-around manner. For example, if one chain sequentially transmits samples 1-64, then a second chain sequentially transmits samples 64 and 2-63. Note that this cyclic shift can be independently applied to samples from each field.
In summary, a cyclic shift means that the same sample is transmitted at different times in the different chains. In one embodiment, the amount of cyclic shift can be measured by the time differential between the time the sample is originally designated to be transmitted and the time that sample is actually designated to be transmitted. The 802.11n standard specifies the cyclic shifting delay (CSD) for two, three, and four chains.
For example, the 802.11n standard specifies the cyclic shift amount for a first CSD 212 (which is applied to legacy header 210 and to HT signal fields 205/206 of HT header 211) to be 0 and −200 ns on chains 0 and 1 (two chains), and 0, −100 and −200 ns on chains 0, 1, and 2 (three chains). Additionally, the 802.11n standard specifies the cyclic shift amount for a second CSD 213 (which is applied to HT short training field 207 and HT long training field 208 of HT header 211) is 0 and −400 for chains 0 and 1 (two chains), and 0, −400, −200 on chains, 0, 1, and 2 (three chains). These cyclic shifts create artificial multipath in the channel as observed by the receiver.
Notably, this cyclic shifting poses significant challenges to accurately determining timing offset. Specifically, as described above for a legacy packet, any timing offset may be characterized as caused by the receiver. However, in the case of a mixed mode packet, samples are intentionally being delayed during transmit. Unfortunately, the receiver does not currently have the information necessary to determine if such intentional delays are being added at the transmitter. That is, the receiver does not know if a received packet is a mixed mode packet or a legacy packet. As a result, the conventional techniques used for estimating receiver offset when receiving a legacy packet cannot be used when receiving a mixed mode packet.